Awesome
I didn't get to play the beta all that much, and I can't really give this game a good play since I'm stuck on my laptop and the little touch-pad is QUITE inadequate for aiming and firing simultaneously.
Anyway, I can comment on how I liked the game's overall smoothness, and the graphic's cleanness. It's well put together, and I noticed you'd mentioned some other ideas you had in store for the future that I'll be sure to check out.
Seriously though, zoom difference upgrades would be cool. Consider building it into any defense-game you might make in the future.